1. Field of the Invention
This invention relates to a dielectric resonator antenna system with wide bandwidth and, in particular but not exclusively to, such a system for use as an element in a phased array.
2. Discussion of Prior Art
The dielectric resonator antenna is well known. It may be probe fed (e.g. S. A. Long, M. W. McAllistar and L. C. Shen; IEEE Transactions on Antennas and Propagation AP-31, No 3, May 1983, pp406-412 and S. A. Long and M. W. McAllistar; International Journal of Infrared and Millimeter Waves, 7, No4, 1986, pp550-570) where the probe has length approximately equal to one quarter of the operating wavelength, and is used to excite a fundamental mode in a coupling block which takes the form of a dielectric puck. The dimensions of the puck are such that it resonates at a specific frequency, this frequency being determined, to a large extent, by the overall volume of the puck.
Alternatively the coupling block may be excited using a patch antenna formed from microstrip, a form of waveguide comprising a copper strip separated from a groundplane by a dielectric substrate. The copper strip is etched to leave an antenna of the required shape and size, typically a square patch fed at the centre of one edge and with the length of each edge equal to half the operating wavelength. Such antennas have the advantage that they occupy little space and can be conveniently connected to form thin planar arrays.
In an array, each element has its own input and output and by varying the phase of the signal at each element the array can be arranged to transmit or receive in a chosen direction. Moreover the chosen direction can be made time dependant so that a given field can be scanned.
At the interface between the coupling block and air, some of the signal is reflected rather than transmitted. This loss of power can be minimized by including an antireflection layer between the dielectric layer and the air (e.g. British Patent Publication no. GB 2 248 522 A). In order to minimize reflection between two media, the thickness of the antireflection layer should approximate to a quarter wavelength of the signal being transmitted. In addition the material of the antireflection layer should (in theory) have a dielectric constant which approximates to the geometric mean of the dielectric constants of the media on either side. In practice, considerable departure from this ideal is acceptable: for example for matching between air (dielectric constant=1) and a coupling block of material with dielectric constant=10, the ideal matching material would have a dielectric constant of 3.16. In practice it is found that polymethylmethacrylate with a dielectric constant of 2.4 serves adequately as a matching material.
Although the foregoing configurations are relatively simple, their use is limited by the inherently narrow range of frequencies over which they can be operated (ie their inherently narrow bandwidth). For example, H. LI and C. H. CHEN describe a probe fed antenna with bandwidth of approximately 200 MHz at 20 dB in Electronics Letters vol. 26 No. 24 (22 Nov. 1990) pp2015-2016.